Such laser sources are in particular used for laser-material interactions consisting for example in accelerating particles (protons, electrons, ions) or in generating secondary radiation in the region of far-UVs, X- or γ-rays. The pulses are focused on a generally solid target with the aim, for example, of creating a plasma at the surface of the latter.
These laser sources with very high peak power are essentially based on solid-state laser source technologies and the principle of chirped pulse amplification, whether this involves amplification by laser effect, or “CPA”, the acronym for the expression Chirped Pulse Amplification, or amplification by non-linear optical effect such as the amplification of parametric fluorescence or “OPCPA”, the acronym of the expression Optical Parametric Chirped Pulse Amplification.
From the moment that lasers with very high peak powers are used, it is necessary to pay a great deal of attention to the problem of temporal contrast of the pulses. Indeed, given the multiple-amplifier configurations of these laser sources with high peak power, there always remains a residual light signal generated by the amplification of the spontaneous emission (or “ASE”) in the case of laser amplification (CPA) or by the amplification of the parametric fluorescence in the case of parametric amplification (OPCPA). This parasitic signal shown in FIG. 1 has a much larger temporal width than the main pulse; it typically exists during the pumping pulses, which generally have pulse durations of about a few nanoseconds. And most importantly, it is already present before the main pulse. The temporal contrast is the ratio between the peak power P1 of the main pulse and the peak power P0 of the ASE pulse; the target values and the measurements for P0 are generally situated in a time interval between 50 and 100 picoseconds before the main pulse, in such a way as to remove other effects such as imperfect compression. The problem is the same if the ratio is very high, in the case of a main pulse of very high peak power, the ASE pulse will still have a significant peak power. If one considers the case of a main pulse of 1 PetaWatt with a temporal contrast of 109:1 (P1/P0=109), the ASE will have a peak power of 1 MegaWatt, enough in the case of focusing on a target, to create a plasma on the latter (notably if it is solid) before the arrival of the main pulse which is disastrous for the intended applications.
It is therefore necessary to optimize the temporal contrast of the pulses, knowing that to obtain satisfactory results with any solid material forming the target, it is necessary to aim for temporal contrasts with respect to the ASE of the order of 1012:1 for a laser of 1 petawatt and of 1013:1 for a laser of 10 petawatts.
Until now, the technologies used to increase the temporal contrast are:                the addition of a saturable absorber, or        the implementation of a non-linear filter based on the technology of generating a cross-polarized wave in a non-linear crystal, subsequently denoted by “XPW” for Cross-Polarized Wave, or        the use of plasma mirrors.        
A saturable absorber is very simple to implement but its contribution is limited because it does not make it possible to improve the contrast of the pulses by more than one or two orders of magnitude. This is mainly due to the fact that the laser damage threshold of these materials are relatively low.
The XPW filter technique was demonstrated for the first time in a laser chain with high peak power in 2004. The architecture of a XPW filter is relatively simple but its efficiency is not very high (at the output a maximum of 30% of the energy of the input pulse is obtained) and the theoretical increase in contrast (output contrast=cube of the input contrast) is heavily limited by the extinction ratios of the polarizers used, which means that the net gain is only of 4 to 5 orders of magnitude, which is still clearly better than the saturable absorber. A XPW filter includes two polarizers and one or two non-linear crystals between the two polarizers.
Plasma mirror technology has been used for a few years now to improve the contrast of laser chains with high peak power. The principle is based on the use of the beam at the output of the chain, therefore after the final temporal compression. The beam is focused on a transparent medium; the ASE pulse is therefore transmitted but from the start of the main pulse there is enough intensity to create a plasma at the surface of the transparent material. This plasma is reflective, thus forming a plasma mirror, and it will reflect around 70% of the main pulse which will be “rid” of a large part of the ASE pulse which will have been transmitted before the creation of the plasma. However, it will be necessary to repeat the operation a second time to obtain an increase in the contrast of around 4 to 5 orders of magnitude. This main pulse, reflected twice, is then focused on the target.
This technology has several drawbacks. The energy loss is therefore of the order of 50% and it is definitive since there are no further amplifiers afterwards, unlike in the case of the XPW filters. Moreover, this technology is relatively complex to implement. Firstly, the assembly of the device is under vacuum since it involves a compressed beam and the assembly is quite bulky given the size of the beam. Secondly, it is obviously necessary, after each shot, to move the plasma mirrors since the light spot of the focused laser has produced a highly reflective plasma but has also locally produced irreversible damage to the optical surface. This therefore entails the installation of precise motorized parts that are compatible with the vacuum.
To obtain temporal contrasts of the order of 1011:1 or even of 1012:1 for a solid target, at the minimum a few tens of picoseconds before the main pulse, none of these techniques considered stand-alone is sufficient and it will therefore be necessary to combine them. Due to this fact, given the relatively small contribution of saturable absorber technology to the improvement of contrast, a combination of the saturable absorber with one of the two other techniques cannot suffice insofar as the “natural” contrast at the start of the laser chain before the use of these devices is of the order of 105 to 106:1. It is therefore necessary to combine a XPW filter and a double plasma mirror, to obtain the required level of contrast, knowing that the latter device has the aforementioned drawbacks.
As a consequence, there remains to this day a need for a system simultaneously satisfying all the aforementioned requirements in terms of peak power, temporal contrast, energy, and simplicity of implementation.